Electrical incandescent filament devices

ABSTRACT

An electrical device with an elongated tubular envelope has an incandescent filament supported along the axis of the envelope by ceramic plates mechanically bonded to the filament and fitting closely in the envelope. The plates may be interconnected by a link rod parallel to the filament which is a single-ended lamp serves also as a lead to one end of the filament.

United States Patent 111111111(IIII/flllIlIlI/Tj Coaton Dec. 30, 1975[54] ELECTRICAL INCANDESCENT FILAMENT 2,813,993 11/1957 Fridrich 313/274X DEVICES 2,901,658 8/1959 Pero et al. 313 275 x 2,910,605 10/1959 Hodge313/275 X [75] In n r: Jam ich r a n, L n, 2,980,820 4/1961 Brundige etal. 313 275 x England 3,195,001 7/1965 Hodge 313/274 X [73] Assignee:Thorn Electrical Industries Limited,

London England Primary ExaminerSaxfield Chatmon, Jr. [22] Filed: July 5,1974 1 Attorney, Agent, or FirmRobert F. OConnell [21] Appl. No.:485,778

[30] Foreign Application Priority Data [57] ABSTRACT July 10, 1973United Kingdom 32729/73 An electrical device with an elongated tubularenve- [52] U.S. Cl. 313/275; 313/277; 313/222; lope has an incandescentfilament supported along the 313/315 axis of the envelope by ceramicplates mechanically [51] Int. Cl. HOIJ 1/90; l-lOlJ 19/44 bonded to thefilament and fitting closely in the enve- [58] Field of Search 313/274,275, 277, 276, lope. The plates may be interconnected by a link rod313/279, 222, 315 parallel to the filament which is a single-ended lampserves also as a lead to one end of the filament. [56] References CitedUNITED STATES PATENTS 5 Claims, 5 Drawing Figures 1,997,975 4/1935Perrin et al 313/275 X Sheet 1 of 2 3,930,17

US. Patent Dec. 30, 1975 ELECTRICAL INCANDESCENT FILAMENT DEVICESfilament lamps. Examples of such devices include linear tungstenhalogenlamps and linear infra-red heater lamps. These lamps have hermeticallysealed, narrow bore tubular envelopes containing filaments which extendaxially along their envelopes. Commonly, the filaments are coiled.

One of the main problems restricting the successful development of suchlinear devices has been the difficulty of supporting their filamentsadequately within their envelopes. In certain linear lamps, e.g.infra-red lamps, the lamp filament is supported by thin refractory metaldiscs, e.g. of only several thousandths of an inch thickness, which arespaced at suitable intervals along the lamp. The filament passes throughan aperture in the centre of each disc. The disc has a diameter onlyslightly less than the envelope bore so that this arrangement locatesthe filament centrally along the axis of the tubular envelope. Thesupport discs are usually made from an active metal, such as tantalum,niobium, titanium etc. which acts as a getter for residual impuritieswithin the lamp envelope. In an alternative construction, spiral wiresupports are used. The wire supports each have a few inner turns wrappedaround the filament, an outer turn or turns engaging the inner wall ofthe envelope, and a spiral of wire linking the inner and outer turns.This arrangement is generally used to support the filament of a lineartungsten-halogen lamp, tungsten wire being used to avoid chemicalreaction with the halogen and/or halides present in the lamp. Spiralwire supports are also used in conventional linear incandescent lamps,wherein the material used for the support is frequently chosen to act asa getter in the finished lamp.

With both of the above types of filament support it is possible for thesupport to become tilted or skewed relative to the tube axis. Toovercome skewing, the lamp designer has relied upon the coiled filamentbeing sufficiently stiff to prevent the supports from tilting inside thetubular envelope. This effectively sets a lower limit on the filamentpower rating, and this is particularly restrictive for mains voltagedevices. As an example, this limit corresponds approximately to 300W for240V lamps with 2000b rating.

We have found that by using self-supporting filament supports made froma ceramic material it is possible to avoid undue local stressing of thefilaments, which thereby makes practicable linear lamps of lowerfilament ratings than hitherto available.

According to the present invention there is provided a linearincandescent filament electrical device having an elongated tubularenvelope containing a filament extending along the tube parallel to thecentral axis thereof, the filament being supported, at intervals alongthe tube, by transversely-directed ceramic plates to which the filamentis bonded mechanically, each plate being a snug fit within the envelope.Preferably, a coiled-coil filament is used, and it may be centred uponthe central axis of the envelope. Each plate is preferably of suchdimensions that it is unable to become skewed relative to the axis ofthe tube. For the ceramic plates to be stable within the envelope, it isdesirable that their edges,-which engage the inside surface of theenvelope, be square to the planes of the plates. It is found that a verystable arrangement is obtained if the plates are of the order of 6.6 mm.in diameter and 0.5 mm. in thickness, when the envelope has a nominalinside diameter of 7 mm.

The rigidity of the structure within the envelope, namely, the filamentand the ceramic plates can be enhanced by interconnecting the plates byone or more link rods. Conveniently, the link rods are also bondedmechanically to the ceramic plates. The use of such link rod(s)considerably stiffens and stabilises the structure and may be preferred.If desired, the ends of the link rod(s) can be made to engage theenvelope to restrict or prevent axial movement of the structure withinthe envelope.

The link rod can also serve as an electrical lead connected to one endof the filament and extending to the opposite end thereof to give asingle-ended lamp in which both filament leads emerge from the same endof the envelope. To prevent arcing the electrically-conductive link rodcan be provided with an insulating coating or sheath over at least partof its length.

The mechanical bonding can be regarded as a shrinkfit of the ceramicplates on to the filament obtained during a sintering operation whichprecedes the installation of the filament and the ceramic plates withinthe envelope. In some instances, some of the outer turns of the filamentmay actually become embedded in the ceramic plate. In addition, theceramic material may become keyed to draw markings" on the surface ofthe filament wire. Y

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 is a partial side elevation of a linear incandescent lampembodying the invention and includes a view taken along the line A-A;

FIG. 2 is a view,-similar to FIG. 1, illustrating an intermediate stagein the manufacture of the lamp shown in FIG. 1;

FIG. 3 shows plan views of several filament supports which may be usedin the lamp shown in FIG. 1;

FIG. 4 is a side view of a lamp similar to that of FIGS. 1 and 2 butwith a link rod which engages the ends of the envelope, and

FIG. 5 is a side view of a single-ended linear incandescent filamentlamp in accordance with the invention.

The lamp shown in part in FIG. 1 is a linear incandescent lamp. It can,for example, be a conventional filament lamp such as an infra-red lamp,or a tungstenhalogen lamp. In the latter instance, the lamp may possessa gas filling to 10 atmospheres pressure of a mixture of kryptoncontaining pg of CH]; and I5 pg of PNBr Gas fillings of this type aredisclosed in our British Patents Nos. 1,236,174 and 1,318,71 1. Whilstthe lamp construction shown in FIG. 1 is suitable for low power, longlife lamps operating directly from the normal mains supply, the actuallamp rating is unimportant.

The lamp has an envelope 10, for example of alumino-silicate or highsilica glass, which is of elongated tubular shape. A coil filament 11extends along the length of the envelope 10 parallel to or coincidentwith the central axis of the envelope 10. The filament has tails 12which in practice are electrically connected by lead-in conductors toexternal terminals, and the envelope has hermetic seals, for instancepinch seals at its ends. The nature of the electrical connections andthe seals do not form part of this invention and accordingly detailsthereof are omitted.

Spaced at intervals along the filament l l are filament supports 14 inthe form of plates of ceramic material. The ceramic material ispreferably an oxide, such as alumina, magnesia, yttria, thoria or commonspinel, because these materials can easily be fabricated using wellestablished techniques and remain stable at high lamp operatingtemperatures. The filament 11 passes through each of the ceramic plates14 and is bonded mechanically thereto. The bond is effectively ashrinkfit of the plate upon the filament. In some cases, the filamentmay become partially embedded in the plate.

The thickness and configuration of the ceramic plates 14 are chosen sothat they are self-supporting inside the tubular envelope 10, and hencethe plates hold the filament 11 in its correct position without relyingupon the mechanical strength of the filament to hold the plates 14 inplace. Also, the thickness and configuration are chosen so that theoperating temperature at the point of contact between the ceramic plate14 and the filament l 1 is below the melting point of the ceramic. Aswill be seen, each plate has a substantial thickness and its peripheraledge is normal to the plane of the plate. The plate 14 is a snug fitwithin the bore of the envelope, and its periphery comprises the majorportion of a circle. A minor segment is missing from the circular plate14, thereby leaving a gap between-the periphery of the plate 14 and theinnersurface of the envelope 10. This gap assists thorough exhaustionand gas filling during manufacture.

- In a particular example of a lamp constructed as shown in FIG. 1, theenvelope had a nominal internal diameter of 7 mm, .the filament coil hadan outside 1 diameter of 0.76 mm and the ceramic plate had a diameter of6.6 mm. Its thickness was approximately 0.5 mm. It will thus be seenthat the plate 14 is somewhat smaller than thenominal bore of theenvelope 10 so as to allow for the dimensional'tolerance on the bore.Nevertheless, it is found that the plates 14 will stand within theenvelope 10, as shown, without tending to tilt or skew relative to thecentral axisof the envelope 10. The difference of size does mean thatthe plates 14 can roll to and fro slightly within the envelope 14, butsuch movement only shifts the filament slightly in a transversedirection, without stressing the filament significantly. Undue stressingof the filament 11 in the region of each plate 14 is thus avoided.

The rigidity of the internal structurewithin the envelope.l0 canoptionally be improved by means of a rod 15 linking the ceramic plates14. The rod 15, like the filament 11, is bonded mechanically-to theplates 14. The rod 15 can be metallic or ceramic, and significantlystrengthens the internal structure. Whilst only one'rod 15 is shown,additional stiffening rods parallel to the filament 11 could beemployed. The rod or rods 15 can be made from high purity densealumina,1 mm.

in diameter. l

The method of making a linear lamp' embodying the invention, which hasan elongated tubular envelope containing a filament extending lengthwisealong the envelope, comprises the steps of fabricating apertured supportplates from a ceramic material such that in the unsintered state theplates are oversize relative to the envelope bore, threading thefilament through a plurality of the support plates and spacing them atintervals along the filament, and firing or sintering the assembledplates and filament so as to cause the plates to contract about thefilament and to contract sufficiently to fit snugly within the envelope,the assembly thereafter being installed in the envelope. Usually, arefractory metal mandrel is threaded through the filament to prevent sagand distortion during the ceramic sintering process, and thisis removedafter sintering. In addition, a ceramic or metallic linking rod isdesirably threaded through, or otherwise attached, to the plates priorto firing or sintering, so as to become mechanically bonded to theplates to rigidify the structure.

FIG. 2 shows the assembly of plates 14, filament 11 and link rod 15 forthe lamp shown in FIG. 1, prior to firing. It will be seen that theplates 14' are of similar shape to the plates 14 of the finished lamp,having peripheral edges square to the planes of the plates and beinglarger than the finished plates 14. At this pre-firing stage, thediameter of the plates 14' is 8.2 mm compared with the 6.6 mm diameterof the plates 14 after firing. Each plate has two apertures 16, 17 toreceive the filament and link rodyIt is found that the apertures 16, 17should be approximately 5% larger in diameter than the-parts 11, 15 theyare to accommodate. The actual diameter is not critical provided that itis greater than the outside diameter of the coiled filament 11 beforesintering but less than that after sintering. Satisfactory results havebeen obtained when the platescontract by approximately 20%.

The ceramic plates 14' are fabricated in the usual way familiar to thoseversed in the art, so as tohave a thickness of 0.63 mm. The plates aremade from A1 0 powder to which 0.05% MgO and 0.O5Y O have been added toimprove sinterability. Three of these plates are threaded on to a 240V Wcoiled coil filament having an outside diameter of 0.76 mm andcontaining a molybdenum mandrel 20 to keep the filament straight. Atthis stage the filament wire is in a fibrous condition.

A number of these assemblies are held in .a suitable jig fabricated frommolybdenum, which isthen loaded into a hydrogen tube furnace forapproximately 2h at 1750C. During this time the ceramic material forms adense polycrystalline structure, shrinks onto the filament 11 and thefilament wire partially recrystallises giving a more stable structure.The final diameter of each sintered plate 14 is 6.6 mm and its thicknessis approximately 0.5 mm. Finally, the molybdenum mandrel is withdrawn ordissolved in nitro-sulphuric acid and the assembly is washed and driedready for installation in the lamp envelope tube. Appropriate lead-inconductors are connected to the filament tails l2 and the envelope 10 isthen evacuated and gas filled in the usual way, using our gas fillingsexemplified hereinbefore in the case'of a tungsten halogen lamp.

If desired, getters may be applied by attaching an active metal to thesupport arrangement or by applying a getter in suspension or solution tothe filament or support assembly, prior to installation in the envelopetube. For a conventional non-regenerative lamp, the getter can betantalum.

FIG. 3 of the drawings illustrates the profiles of various alternativesupport plates. The plate 30 is wholly circular, and has two apertures.One aperture is to accommodate the filament and the other to assistthrough exhaustion and gas filling. A third aperture may be provided ifseveral such plates are to be connected by a link rod. The remainingplate profiles, as shown in FIG. 3, are not intended necessarily to beconnected by link rods. Nevertheless, one or more apertures can beprovided in these alternative plates to accommodate a correspondingnumber of link rods, if required.

The ceramic support plates are considerably cheaper than conventionalmetallic supports. It would be wholly impractical to make refractorymetal supports of comparably massive thickness, and not only on theground of expense. A major failing of a thick metal support would lie inthe amount of heat which it would conduct away from the filament. Thiswould, inter alia, cause severe local filament cooling, unduly heat theenvelope with the attendant risk of envelope failure, and reduce thelight output, hence efficacy, of the lamp. The ceramic support platesare quite poor heat conductors, and by appropriate choice of theirprofiles, their cooling effects upon the filament can be largelycontrolled. The ceramic plates are further of advantage in that, unlikerefractory metal supports, they can be accurately pressed, and requireno further machining or deforming prior to attachment to the filament.

FIG. 4 of the drawings shows a modification of the lamp of FIGS. 1 and 2in which corresponding parts have been given the same referencenumerals. In this construction the link rod is extended so that its ends21 engage the end walls 22 of the tubular envelope 10. The link rod 15thus not only improves the stability of the structure and maintains thecorrect spacing between the ceramic suppo'rts but also prevents axialmovement of the structure so that the filament and support assembly ismechanically stable at every burning angle.

In the single-ended lamp of FIG. 5 a filament 31 is supported along theaxis of a tubular envelope 32 by ceramic'plates 33. The composition andmethod of assembly of these parts and the lamp fill may be as describedwith reference to FIGS. land 2. The plates 33 are joined by a link rod34 which in this case is an electrically-conductive metal rod. One 'endof the link rod 34 is bent over to form an arm 35 to which a tail 36 ofthe filament 31 is attached. The other end of the link rod 34 forms anelectrical lead 37 which passes through the-end of the envelope 32remote from the filament tail 36. The filament 31 has a second tail 38which is 5 joining the leads 37 and 39. In this way the filament,

together with its leads and supports, forms a single rigid assemblysecured together by the link rod 34. The manufacture of the lamp is thusgreatly simplified. After the assembly has been constructed and sinteredit can be inserted in the envelope, which is then sealed and exhaustedin a conventional manner.

In some cases it is necessary, in order to avoid arcing, to isulate atleast part of the link rod 34 which carries current to the filament 31.This can be achieved by the use of a glass or ceramic coating or, asshown in FIG. 5, by threading insulating tubes 41 over the rod 34between the support plates 33.

I claim:

'1. In a linear incandescent filament electrical device having anelongated tubular envelope containing a filament extending along thetube parallel to the central axis thereof and supported at intervals bysupports which engage the wall of the envelope, the improvement whereinthe supports are plates of electrically- 'insulating, ceramic materialheat-shrunk onto the filament to form amechanical bond therewith, eachplate having edges which fit closely against the inside surface of theenvelope and are of sufficient width to prevent skewing of the platerelative to the axis of the envelope.

2. In an electrical device as claimed in claim 1, wherein theimprovement further includes at least one link rod extending parallel tothe axis of the envelope and interconnecting the plates.

3. An electrical device as claimed in claim 2, in which each end of thelink rod engages with the ends of the envelope to prevent axial movementof the supports.

4. An electrical device as claimed in claim 2, in which the link rodforms part of an electrical lead which is connected to one end of thefilament and extends by way of the link rod to the opposite end of thefilament, and-passes through the same end of the envelope as a leadconnected to the said opposite end.

5. An electrical device as claimed in claim 4, in which theelectrically-conductive link rod is provided with an Disclaimer3,930,178.James Richard Coaton, London, England. ELECTRICAL INCAN-DESCENT FILAMENT DEVICES. Patent dated Dec. -30, 1975. Disclaimer filedFeb. 8, 1985, by the assignee, Thorn EMIpIc.

Hereby enters this disclaimer to all claims of said patent.

[Official Gazette April 8, 1986.]

1. In a linear incandescent filament electrical device having anelongated tubular envelope containing a filament extending along thetube parallel to the central axis thereof and supported at intervals bysupports which engage the wall of the envelope, the improvement whereinthe supports are plates of electricallyinsulating, ceramic materialheat-shrunk onto the filament to form a mechanical bond therewith, eachplate having edges which fit closely against the inside surface of theenvelope and are of sufficient width to prevent skewing of the platerelative to the axis of the envelope.
 2. In an electrical device asclaimed in claim 1, wherein the improvement further includes at leastone link rod extending parallel to the axis of the envelope andinterconnecting the plates.
 3. An electrical device as claimed in claim2, in which each end of the link rod engages with the ends of theenvelope to prevent axial movement of the supports.
 4. An electricaldevice as claimed in claim 2, in which the link rod forms part of anelectrical lead which is connected to one end of the filament andextends by way of the link rod to the opposite end of the filament, andpasses through the same end of the envelope as a lead connected to thesaid opposite end.
 5. An electrical device as claimed in claim 4, inwhich the electrically-conductive link rod is provided with aninsulating coating or sheath.